A variable impedance output buffer is designed so that the output impedance of the buffer is set relatively low during an initial portion of an output transition and subsequently the output impedance of the buffer is increased near the end of the output transition to more closely match the characteristic impedance of a transmission line driven by the buffer. The output impedance is changed when a buffer output voltage crosses a predetermined threshold. It is important that the circuit is set to trigger the change of output impedance near the end of an output transition in a reliable and consistent manner. Known voltage threshold detectors that implement this variable impedance function do not always offer as reliable and consistent a detection threshold as is desired in state-of-the-art integrated circuit technologies. This can adversely affect transmission line performance.